1. Field of the Invention
The present invention relates to a high-flex insulated electrical cable, and in particular to such a cable having a plurality of multi-strand layers arranged in superposed or side-by-side condition and disposed in long lay at the same angle of twist.
2. Prior Art
For example, in the case of flat knitting machines in which the needle selection is effected electronically problems are encountered in relation to the electrical cable which is fixedly anchored at one end midway along the machine frame and at the other end to a recriprocating carriage. During reciprocation of the carriage the cable is not only bent but also twisted. In particular it has been found that the resulting torsional stress is more damaging to the cable than the simple bending stress, even more so in a knitting machine arrangement than in other technical arrangements, for example the arrangements found in traction or conveyor plants.
Electrical cables having concentric layers of strands laid in reverse with one another are known. This arrangement is of great disadvantage not only for withstanding the bending stresses but also the torsional stresses to which the cable is subjected. This is so because in the case of torsional stressing the strands of one layer are twisted together and those of the other apart, i.e., are respectively compressed and stretched. Counter twisting is therefore necessary in such cables if they are to retain their shape.
In cables of this nature, which are exposed to bending stresses, it has been proposed to dispose the concentric layers of strands in long-lay (see for example, German laid open application No. 1,465,777), an attempt being made to keep the cable in shape by winding the banding between the layers with a reverse twist. In the case of cables which are subject to bending stresses only, this may be of some advantage in relation to the counterlaying of the layers despite the reverse twisting of the banding. This does not apply, however, to cables which are exposed to torsional stresses in addition to, or in place of, the bending stresses. Thus, where there is reversed-direction banding of this kind, torsioning of the cable leads to shifting of the strand layers relatively to the banding, leading to damage of the insulation of the individual strands and possibly even to breakage of the cable or of individual strands. In the case, for example, of knitting machines which are in operation continuously, the frequency of the reciprocating movement of the carriage is very high, for instance in the range of some 10 passes per minute, and this must be accounted for in addition to the above noted considerations.